tst_QDnsLookup: don't add duplicate 'nameserver' lines

I don't know why systemd-resolved did have those for me, but it did, so
QtTest complains.

QWARN  : tst_QDnsLookup::setNameserver() Duplicate data tag "..." - please rename.

Pick-to: 6.8
Change-Id: I8df70eb429682b529ecefffd0aca735f94d77225
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

tests/auto/network/kernel/qdnslookup/tst_qdnslookup.cpp

@@ -128,8 +128,9 @@ static QList<QHostAddress> systemNameservers(QDnsLookup::Protocol protocol)
             if (!line.startsWith(command))
                 continue;
 
-            QString addr = QLatin1StringView(line).mid(sizeof(command));
-            result.emplaceBack(addr);
+            QHostAddress addr(QLatin1StringView(line).mid(sizeof(command)));
+            if (!result.contains(addr))
+                result.emplaceBack(std::move(addr));
         }
     };
     parseFile("/etc/resolv.conf"_L1);